If water is entering via the washer jet base then the only real lasting cure is to drop the dash, release the pipes and unscrew each jet, replace the little outer washer with a tiny rubber one or perhpas an O ring and refit everthing back, whilst in there check the quality of the washer pipework as some have found it becomes brittle and leaks (I suspect the ethanol in winter screen wash additive rots the tubing)
